1. Field of the Disclosure
The disclosure relates generally to frequency synthesis devices and, more particularly, to frequency synthesis using MEMS (microelectromechanical systems) resonators, or microresonators, for reference signal generation.
2. Brief Description of Related Technology
MEMS resonators are attractive for use in many applications as a cost-effective replacement for discrete devices such as quartz crystal oscillators or surface-acoustic wave (SAW) resonators. MEMS resonators are particularly promising for use in wireless communications systems, as they can be fabricated alone or on substrates with other circuitry, such as MOS or bipolar circuits. MEMS resonators can also have very high mechanical quality factors (Q), which leads to good frequency selectivity. MEMS resonators also typically consume less power than their discrete counterparts.
MEMS resonators are not without drawbacks, however. The center frequency of a MEMS resonator is determined by its physical characteristics, which are, in turn, functions of design, materials, and the processing methods used to fabricate the resonator. Due to its high-Q nature and the normal process variations that occur in fabrication, it is difficult to fabricate a MEMS resonator with a center frequency accuracy of better than a few percent.
Many applications for which MEMS resonators are well suited demand initial accuracy of between 1 and 100 parts-per-million (ppm), which is 3 to 5 orders of magnitude more precise than typical accuracy. In order to reach the requisite level of accuracy, laser trimming or other methods have been used. Trimming methods have generally been found to undesirably add to the complexity and cost of the fabrication process. Thus, despite the number of different trimming methods available, it would nonetheless be desirable to develop an alternative for achieving the necessary 1 to 100 ppm center-frequency accuracy that does not involve trimming and other complex fabrication steps.
In addition to initial frequency inaccuracy, the resonant frequency of a MEMS resonator is dependent on temperature. The temperature dependency of the resonant frequency can be as much as 17 ppm/° C. Unfortunately, the maximum allowable temperature variation is 0.02 to 1 ppm/° C. (a difference of 4 orders of magnitude) for many applications. Several methods for achieving temperature compensated MEMS resonator structures have been proposed, but these proposals have all required additional processing steps during resonator fabrication. The cost and complexity added by these processing steps make these approaches unattractive.
A method of adjusting the initial frequency of a MEMS resonator and compensating for temperature-change-induced frequency variation without the need for extra manufacturing steps during fabrication would be desirable. Such a method would reduce the cost and manufacturing complexity associated with producing a MEMS resonator product.